feat(transport): add value codec

[rvolosatovs]

• Remove associated Error types, and use anyhow::Error in wrpc_transport traits to simplify downstream usage. I really wish there was a nicer way to handle this and std::io::Error is a real pain to use in this context
• Add ValueEncoder and ValueDecoder drafts, the implementations will have to be expanded, but the idea is that users of wrpc_transport will implement Encoder and Decoder on these structs